Method to produce a metal strip, and production plant implementing said method

ABSTRACT

Method to produce metal strip that comprises the casting of a cast product through a casting machine provided with a crystallizer to obtain a slab, and the hot rolling of the slab in a rolling station to obtain metal strip. The casting machine, during casting, exerts an action of reducing the thickness of the cast product exiting the crystallizer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application number 17/283,820, filed Apr. 8, 2021, which is the U.S. national phase entry of PCT/IT2019/050210, with an international filing date of Sep. 25, 2019, which claims priority to Italian Application No. 102018000009259, filed Jan. 8, 2018, the entire disclosures of which are fully incorporated herein by reference.

FIELD OF THE INVENTION

The present invention concerns a method to produce a metal strip and the production plant implementing said method.

In particular, the method according to the present invention allows to define the modes to obtain a metal strip and a constructive layout of a plant for producing hot-rolled metal strip.

BACKGROUND OF THE INVENTION

In the iron and steel industry, plants for producing metal strip are known which usually comprise a mold configured to cast slabs, extraction devices provided to extract the slabs from the mold, and a rolling line located downstream of the extraction devices, configured to reduce the overall thickness of the slab until a metal strip of the desired thickness is obtained.

Depending on the thicknesses of the metal strip to be obtained, as well as on the overall productivity that the plant needs to have, it is known to suitably size the entire plant so that it complies with the required parameters, at least with regard to the productivity of the plant and the thicknesses of the strip.

In relation to these requirements, it is known to supply the client with “endless” -type plants, semicontinuous-type plants, for example “coil to coil” and/or “semi-endless”, or “endless” and semicontinuous combined plants.

The endless-type plants provide to supply the cast product, gripped directly, from the mold to the rolling line without cutting the product being worked except for before the final winding.

The semicontinuous-type plants provide that downstream of the casting machine or the roughing stands, the cast products are cut to size and disposed in a heating and/or maintenance furnace which also acts as an accumulation buffer for the cast products when it is necessary, for example, to interrupt the downstream rolling due to small accidents or programmed roll change.

If the cast product is cut, downstream of the casting machine or of the roughing stands, to a length such as to obtain, at the end of the rolling process, a single coil, the process is called coil to coil. If, on the other hand, the cast product is cut, downstream of the casting machine or of the roughing stands, to a length such as to obtain, at the end of the rolling process, a plurality of coils, usually between two and five, the process is called semi-endless.

It is also known that, by means of suitable expedients, a semicontinuous type plant can be made to also function in endless mode, obtaining the advantages of this solution.

The choice of the type of plant to adopt, as well as the number of components required, for example the number of rolling stands, or the choice of how many roughing stands and how many finishing stands to adopt, is usually determined based on the experience of the technicians of the field.

This sizing, that is, the preparation of the strip production plant, however, sometimes does not allow to reach an effective compromise between the investments required for the construction of the plant, also called Capex, and the operating transformation costs, also called Opex. There are therefore situations in which the investments incurred for the construction of the production plant are too high compared to the revenues, and therefore a production plant is supplied that is oversized with respect to the client’s productivity, or alternatively, situations arise in which the production plant is undersized and therefore does not allow to reach the production capacity required by the client.

Some known methods and plants for producing metal strip which however present the problems above are described for example in documents WO 92/00815 A1, JP S62 248542 A and WO 02/40201 A2.

It is therefore a purpose of the present invention to provide a production plant correctly sized according to the needs of the client who works with a method of hot production of metal strip, for example steel, able to optimize the productivity of a strip production plant with the lowest number of stands possible, while maintaining the maximum casting speed in relation to the various types of steel.

It is also a purpose of the present invention to provide a plant for producing a hot-rolled metal strip that requires limited investments to make it (Capex) and has low operating transformation costs (Opex) when compared to a traditional plant for producing the same strip thicknesses.

A further purpose of the present invention is to provide a plant and to perfect the corresponding method to produce a hot-rolled metal strip that can selectively vary the thickness of the cast slab in relation to the final thickness of the strip.

It is also a purpose of the invention to provide a method to produce metal strip which allows to obtain a plant that is extremely flexible and can be adapted to specific client requirements.

It is also a purpose of the invention to provide a plant for producing metal strip that is competitive on the market.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

In accordance with the above purposes, a method to produce metal strip, in accordance with the present invention, comprises the casting of a cast product through a casting machine provided with a crystallizer to obtain a slab, and the hot rolling of the slab in a rolling station to obtain metal strip having different strip thicknesses.

The casting machine, during the casting, exerts an action of reducing the thickness of the cast product exiting the crystallizer.

According to one aspect of the present invention, the method provides that the casting machine is selectively set, on each occasion, as the strip thicknesses vary, in order to exert a different action of reducing the thickness of the cast product.

In particular, the present method comprises, with the sizes of the crystallizer being equal, at least a first step of producing a first strip having a first thickness, wherein the casting machine exerts a first thickness reduction ratio of the cast product, and at least a second step of producing a second strip having a second thickness, less than the first thickness, wherein the casting machine exerts a second thickness reduction ratio of the cast product, different from the first thickness reduction ratio; this thickness reduction ratio is defined as the difference between the thickness of the cast product exiting from the crystallizer and the slab thickness exiting from the casting machine, correlated to the thickness of the cast product exiting from the crystallizer.

This solution allows to modulate the thickness of the slab exiting the casting machine, as a function of the final strip thicknesses to increase the efficiency of the strip production plant and increase the quality of the strip produced.

In particular, with the present invention it is possible in some applications to also reduce the number of rolling stands of the rolling station by at least one unit with the same productivity as a known plant. This determines an economic and efficiency advantage of the entire production plant.

The action of reducing the thickness induced on the strip, produced on each occasion, is divided partly in the casting machine and partly in the rolling station, improving efficiency and increasing the quality of the strip produced.

If, with the same productivity, the number of rolling stands of the rolling station is substantially the same as that of a known plant, with the present invention it is in any case possible to reduce the number of rolling compressions, since part of the reduction in thickness is carried out directly by the casting machine and not only in the rolling station as occurs in the state of the art.

This expedient allows to obtain energy savings thanks to a reduction in the pressure of the compression on the slab being rolled, and to obtain a higher quality of the strip because, for example, the profile and the flatness thereof are improved, and the risk of scale impressed onto its surface is reduced.

Moreover, with the present invention it is possible to reduce maintenance interventions at least in the rolling station.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

FIGS. 1-9 show some of the possible embodiments of a plant for producing a metal strip which implements a method in accordance with the present invention;

FIG. 10 graphically shows some curves, determined in relation to the thickness H of the cast product exiting from the crystallizer, showing the development of the thickness reduction ratio of the cast product exerted by the casting machine as a function of the thickness of the strip;

FIG. 11 shows curves, determined in relation to the thickness H of the cast product exiting from the crystallizer, which show the development of the thickness reduction ratio in the drawing unit as a function of the thickness of the strip;

FIG. 12 shows the criteria for choosing the rolling modes;

FIG. 13 graphically shows a further criterion for choosing the rolling modes in relation to the strip thicknesses and to the capacity of a plant layout to adopt one and/or the other of the rolling modes;

FIGS. 14 and 15 are graphs that correlate the nominal slab thickness to the productivity of the plant and to the casting speed; both charts refer to a casting operation of 7,200 hours/year;

FIG. 16 is a graph which correlates the ratio of thicknesses to the number of rolling stands required;

FIGS. 17-22 show two example embodiments of implementation of the teachings of the present invention.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can conveniently be incorporated into other embodiments without further clarifications.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

We will now refer in detail to the various embodiments of the present invention, of which one or more examples are shown in the attached drawings. Each example is supplied by way of illustration of the invention and shall not be understood as a limitation thereof. For example, the characteristics shown or described insomuch as they are part of one embodiment can be adopted on, or in association with, other embodiments to produce another embodiment. It is understood that the present invention shall include all such modifications and variants.

Embodiments of the present invention concern a method to produce a metal strip N in a production plant 10.

FIGS. 2-9 show possible layouts of plants 10 for producing strip that implement the principles of the present invention.

In particular, hereafter reference will be made to FIG. 1 that shows a plurality of operative components reciprocally disposed along a work line. These operative components shown in FIG. 1 can be combined with each other to obtain one or more layouts for producing strip, then shown in FIGS. 2-9 . For this purpose, the variant embodiments of the production plant 10 shown in FIGS. 2-9 refer to components of a plant described with reference to FIG. 1 .

The plant 10 for producing strip N comprises at least one casting machine 11 in which the liquid metal is cast to be solidified and to obtain a slab B.

In accordance with one aspect of the present invention, the production plant 10 also comprises a rolling station 19 located downstream of the casting machine 11 and configured to hot roll the slab B and obtain strip N

The casting machine 11 comprises at least one mold 15 provided with a crystallizer 15 a in which the formation of a cast product P occurs, which passes through the entire casting machine 11.

The crystallizer 15 a can comprise at least two wide plates facing each other and distanced, at least in the terminal segment, by a determinate value that substantially corresponds to the thickness H of the cast product P exiting from the crystallizer 15 a.

Moreover, the plates of the crystallizer 15 a have a width which is determined in relation to the width of the strip N that is to be obtained.

At exit from the crystallizer 15 a the cast product P has a solidified external shell, or skin, which allows to contain the liquid metal even outside the crystallizer 15 a.

In accordance with some solutions of the present invention, the casting machine 11 can be provided with thickness reduction means located downstream of the crystallizer and which can be selectively set, on each occasion, as the strip thicknesses vary, in order to exert a different action of reducing the thickness of the cast product P exiting from the crystallizer 15 a.

The thickness reduction means can comprise a pre-rolling device with rollers and/or a roller unit, for example of the drawing type as described below.

In accordance with some embodiments, the casting machine 11 comprises a pre-rolling device 16 with rollers, also referred to as roller unit hereafter, located downstream of the mold 15 and configured to perform a reduction in the thickness of the cast product P exiting from the crystallizer 15 a while its core is still liquid or partially liquid.

In accordance with some embodiments of the invention, the roller unit 16 is provided with a plurality of rollers grouped into segments which are opposite each other and aligned along the casting axis.

The rollers are selectively moved toward/away from each other to exert a selective action of reducing the thickness of the cast product.

Regulating devices 17 of a known type, for example hydraulic cylinders, are associated with the segments in order to move the rollers of the roller unit 16 toward/away from each other and perform the reduction of the thickness of the cast product P exiting from the crystallizer 15 a.

The overall reduction of the thickness which the cast product P exiting the crystallizer 15 a undergoes inside the roller unit 16 is a liquid core reduction, therefore it does not generate a lengthening of the material.

The slab B, exiting from the roller unit 16, is completely solidified and has a slab thickness SB1.

The slab thickness SB1 which is obtained at the end of the liquid core pre-rolling determines the productivity of the casting machine 11 and therefore of the entire plant.

The compression carried out by the roller unit 16 allows to join the two half-skins in a joining point, also called “Kissing Point” (KP), in which there is the complete solidification of the cast product. The KP is also seen as the terminal vertex of the liquid cone which originates from the meniscus of the liquid metal in the crystallizer 15 a.

In accordance with one aspect of the present invention, the position of the KP can be varied along the longitudinal extension of the roller unit 16, intervening on the intensity of the liquid core thickness reduction obtained with the adjustment of the segments of the roller unit 16 and on the secondary cooling intensity. The position of the KP is also a function of the casting speed. The thickness of the cast product in correspondence with the KP is equal to the slab thickness SB1 exiting from the roller unit 16.

In accordance with one aspect of the present invention, the slab thickness SB1 is selectively set as a function of the final thicknesses of the strip N, in particular as a function of the product mix to be produced by the plant, as will be better explained below.

In accordance with possible solutions, known types of cooling device, also called secondary cooling, can be associated with the roller unit 16, and configured to cool the external surfaces of the slab and thus determine its further solidification. Cooling devices can comprise nozzles to deliver nebulized water or water mixed with air (“air-mist”). The action of the cooling devices also has an influence on the position of the KP.

In accordance with some embodiments, the casting machine 11 comprises a drawing unit 18 located downstream of the roller unit 16 and configured to draw the slab B toward the rolling station 19.

The drawing unit 18 comprises a number of pairs of drawing rollers comprised between one and six.

The rollers of each pair are disposed on opposite sides of the slab B to be drawn.

In accordance with some embodiments, the drawing unit 18 can also be configured to roll the slab B which passes through it.

By way of example only, the rollers of each pair, or at least one of them, can be associated with movement devices, for example hydraulic cylinders of the known type (not shown), to move the rollers toward/away from each other and also determine a predefined compression action on the completely solidified slab B, as described below.

In accordance with possible solutions, the drawing unit 18 can comprise a first drawing device 18 a located directly downstream of the roller unit 16, that is, in the vertical segment of the casting machine 11. In accordance with one possible solution, possibly combinable with the previous solution, the drawing unit 18 can comprise a second drawing device 18 b located directly downstream of the curved segment of the casting machine 11.

The presence of the first drawing device 18 a is provided only in the case of vertical casting.

The presence of the second drawing device 18 b is provided in the case of vertical and vertical-curved casting.

The first drawing device 18 a and/or the second drawing device 18 b can comprise a number of pairs of rollers comprised between 1 and 3.

As mentioned above, the rollers of the drawing unit 18 are configured to perform a slight liquid core thickness reduction on the slab B of thickness SB1 and therefore a real rolling, albeit of a light magnitude, which determines an elongation of the material. The rolling, albeit light, of the drawing unit 18 generates a further reduction of the thickness of the slab B which takes on a further thickness SB2, smaller than SB1, thus allowing the rolling station 19 located downstream to reach thinner thicknesses. Since the drawing unit 18 is located at the end of the casting machine 11, the thickness SB2 of the slab B exiting from the drawing unit 18 corresponds to the thickness of the slab B exiting the casting machine 11.

The drawing unit 18, together with the roller unit 16 and the crystallizer 15 a, forms an integral part of the casting machine 11 which, thanks to the rolling action performed by the rollers, can be defined as a “Rolling Caster”.

By Rolling Caster we mean a casting machine 11 able to carry out both the casting of the product and also its rolling, partly liquid core and partly solid core.

The reduction in overall thickness which the cast product P exiting from the crystallizer 15 a inside the casting machine 11 undergoes is performed partly liquid core and partly solid core.

According to one aspect of the present invention, the distribution of thickness reduction in the casting machine 11 can be advantageously modulated between liquid core and solid core, according to specific production requirements.

In accordance with possible embodiments, the rolling station 19 comprises a roughing unit 12 configured to roll the slab B.

The roughing unit 12 can be provided with one or more roughing stands 20.

According to further embodiments of the invention, the rolling station 19 comprises a finishing unit 13 configured to roll the slab B and bring it to its final size, that is, define the metal strip N.

The finishing unit 13 is located downstream of the roughing unit 12.

The finishing unit 13, in turn, can also comprise one or more finishing stands 21, each of which is configured to roll and define the strip thickness SN.

According to possible solutions, directly downstream of the roughing unit 12 a cutting shear 26 is provided, configured to cut the roughed slab B and define a bar, also called transfer bar in the sector, which will be subjected to subsequent rolling to obtain the strip N.

The plant 10 can comprise at least one induction heating device, in this case one, two, or three induction heating devices 22, 23, and 34 and configured to heat the slab B.

In accordance with one possible solution, the plant 10 comprises a first induction heating device 22 located downstream of the casting machine 11, in this case upstream of the roughing unit 12, and configured to restore the temperature of the slab before its introduction into the rolling station 19.

According to a possible solution, the plant 10 comprises a second induction heating device 23 located upstream of the finishing unit 13, for example between the roughing unit 12 and the finishing unit 13, and configured to increase the temperature of the bar before its introduction into the finishing unit 13.

In accordance with a possible solution, the second induction heating device 23 is located directly downstream of the roughing unit 12.

According to a further solution, the plant 10 comprises a third induction heating device 34 interposed between two of the finishing stands 21 and configured to restore the temperature of the bar during the finishing process.

In accordance with a further embodiment, the production plant 10 can comprise at least one heating and/or maintenance unit, in this case two heating and/or maintenance units 32 and 33, configured to heat or maintain the temperature of the bar segments.

The heating and/or maintenance unit 32 and 33 can comprise a heating and/or maintenance tunnel type furnace which also acts as an accumulation buffer for the bars in the event the rolling process is interrupted, due to accidents or a programmed roll change, thus avoiding losses of material and energy and, above all, avoiding an interruption of the casting.

According to a possible solution, the production plant 10 comprises a first heating and/or maintenance unit 32 located directly upstream of the rolling station 19.

According to a further implementation, the production plant 10 comprises a second heating and/or maintenance unit 33 located between the roughing unit 12 and the finishing unit 13.

In accordance with this embodiment, it is advantageous to provide that the production plant 10 is provided with a shear 35 interposed between the casting machine 11 and the rolling station 19 and configured to cut the slab B produced by the casting machine 11 into segments. In case of an emergency, or due to maintenance needs due for example to a roll change in the rolling station 19, the segments can be subsequently stored and kept at temperature inside the first heating and/or maintenance unit 32.

In accordance with some embodiments, the production plant 10 comprises an intermediate winding/unwinding device 29 interposed between the cutting shear 26 and the finishing unit 13 and configured to wind the bar cut by the cutting shear 26 and to supply the previously wound and cut bar to the finishing unit 13.

In accordance with possible embodiments, the plant 10 can comprise both the second heating and/or maintenance unit 33 and the intermediate winding/unwinding device 29 located downstream with respect to the heating unit 32.

The intermediate winding/unwinding device 29 can be for example of the type described in the international application WO-A-2011/080300 in the name of the Applicant.

According to some solutions, the intermediate winding/unwinding device 29 comprises a first unit 30 and a second unit 31 which alternate in the function of winding the bar received from the roughing unit 12, and in the function of unwinding the previously wound bar, in order to feed it to the finishing unit 13.

In particular, while one of the two units 30, 31 winds one bar, the other unit 31, 30 unwinds another roughed bar supplying it downstream. This intermediate winding/unwinding device 29 also allows to define a temporary accumulation buffer to compensate for the different work speeds of the casting machine 11 and of the finishing unit 13. In this way, the intermediate winding/unwinding device 29 allows to absorb the shutdown times of the rolling mill for small maintenance or for the programmed roll change or for small accidents/blockages, without needing to interrupt the casting process, and therefore without loss of production and without penalizing the steelworks upstream.

In accordance with possible solutions, the intermediate winding/unwinding device 29 can comprise heating devices, not shown, for heating or maintaining at temperature the bar contained inside.

According to a further solution of the present invention, the plant 10 can comprise a cutting member 28 disposed between the roughing unit 12 and the finishing unit 13 and configured to cut the head end or the tail end of the bar supplied by the roughing unit 12, in this case by the intermediate winding/unwinding device 29, and to be fed to the finishing unit 13.

According to a further solution, a cooling unit 24 is provided between the rolling station 19 and the final winding unit 14, configured to cool the strip N exiting from the rolling station 19 and allow it to be collected in the final winding unit 14.

According to a further implementation of the present invention, the plant 10 comprises a final winding unit 14 of the strip N.

The final winding unit 14 can comprise a winding member 25 suitable to wind the strip N into coils.

In accordance with some embodiments, the plant 10 comprises a cutting device 27 located upstream of the final winding unit 14 and configured to cut the strip N to size, once a predetermined weight of the coil of strip N has been reached. The cutting device 27 can be positioned between the cooling unit 24 and the final winding unit 14.

With reference to FIG. 2 , this shows a variant embodiment of a production plant 10 which comprises, disposed in sequence, the casting machine 11 as defined above, the shear 35, the first induction heating device 22 and/or the first heating and/or maintenance unit 32, the finishing unit 13, the cooling unit 24 and the final winding unit 14.

With reference to FIG. 3 , this shows a further variant embodiment of the production plant 10 which, in addition to the components provided in the production plant 10 of FIG. 2 , provides that the rolling station 19 comprises the roughing unit 12, the second induction heating device 23, the second heating and/or maintenance unit 33, and the cutting member 28 located upstream of the finishing unit 13.

With reference to FIG. 4 , this shows a variant embodiment of a production plant 10 which comprises, disposed in sequence, the casting machine 11 as defined above, the roughing unit 12, the cutting shear 26, the second induction heating device 23, the intermediate winding/unwinding device 29, the cutting member 28, the finishing unit 13, the cooling unit 24 and the final winding unit 14.

With reference to FIG. 5 , the production plant 10 comprises, disposed in sequence, the casting machine 11 as defined above, the rolling station 19 provided with the roughing unit 12, the cutting shear 26, the second induction heating device 23, the second heating and/or maintenance unit 33, the finishing unit 13, the cooling unit 24 and the final winding unit 14.

The production plants 10 described with reference to FIGS. 2-5 can be selectively set to work in coil to coil mode.

With reference to FIG. 6 , the production plant 10 comprises disposed in sequence, the casting machine 11 as defined above, the rolling station 19 provided with the shear 35, the second induction heating device 23, the second heating and/or maintenance unit 33, the finishing unit 13, the cooling unit 24, the cutting device 27 and the final winding unit 14.

The production plant 10 described with reference to FIG. 6 can be configured to work in coil to coil or semi-endless mode.

With reference to FIG. 7 , the production plant 10 comprises disposed in sequence, the casting machine 11 as defined above, the rolling station 19 provided with the roughing unit 12, the cutting shear 26, the second induction heating device 23, the finishing unit 13, the third induction heating device 34 interposed between the finishing stands 21, the cooling unit 24, the cutting device 27 and the final winding unit 14.

The production plant 10 described with reference to FIG. 7 can be configured to work in endless mode.

With reference to FIG. 8 , the production plant 10 comprises disposed in sequence, the casting machine 11 as defined above, the rolling station 19 provided with the roughing unit 12, the cutting shear 26, the second induction heating device 23, the intermediate winding/unwinding device 29, the cutting member 28, the finishing unit 13, the third induction heating device 34 interposed between the finishing stands 21, the cooling unit 24, the cutting device 27 and the final winding unit 14.

The production plant 10 described with reference to FIG. 8 can be configured to work in coil to coil or endless mode.

With reference to FIG. 9 , the production plant 10 comprises disposed in sequence, the casting machine 11 as defined above, the rolling station 19 provided with the shear 35, the first induction heating device 22, the first heating and/or maintenance unit 32, the roughing unit 12, the cutting shear 26, the second induction heating device 23, the finishing unit 13, the third induction heating device 34 interposed between the finishing stands 21, the cooling unit 24, the cutting device 27 and the final winding unit 14.

The production plant 10 described with reference to FIG. 9 can be configured to work in coil to coil, semi-endless, or endless mode.

The embodiments of plants 10 described with reference to FIGS. 2-9 define examples of families of production plant layouts that the end user can choose in an optimized manner as a function of productivity, product mix, and types of steel, also called “steel grades”, which are to be produced.

In accordance with one aspect of the present invention, a method to produce metal strip N comprises at least the casting of a cast product P through the casting machine 11 to obtain a slab B, and the hot rolling of the slab B in the rolling station 19 to obtain metal strip N.

According to one aspect of the present invention, the casting machine 11, during the casting, exerts an action of reducing the thickness of the cast product P exiting from the crystallizer 15 a.

According to a further aspect of the present invention, for each thickness size of the strip N that is produced, the casting machine 11 is selectively set to exert a different action of reducing the thickness of the cast product P exiting from the crystallizer 15 a.

In particular, it is provided that the thickness of the slab B produced by the casting machine 11 will, on each occasion, be regulated as a function at least of the final thicknesses of the strip N to be obtained and possibly of the variety of products, or product mix, to be obtained.

This solution allows to reduce the compression action that has to be exerted in the rolling station 19, to reduce the rolling powers required, to reduce the wear to which the rolling rolls are subjected and in some cases to also reduce, by at least one, the number of rolling stands with respect to known plants that have the same overall productivity.

By way of example only, it can be provided that, given the same sizes of the crystallizer 15 a, the method comprises a first step of producing a first strip having a first strip thickness SN’ wherein the casting machine 11 exerts a first thickness reduction ratio TAU A′ of the cast product P exiting from the crystallizer 15 a, and a second step of producing a second strip having a second strip thickness SN”, less than the first thickness, wherein the casting machine 11 exerts a second thickness reduction ratio TAU A″ of the cast product P exiting from the crystallizer 15 a, and wherein the first thickness reduction ratio TAU A′ is different to the second thickness reduction ratio TAU A″.

In accordance with one solution, the first thickness reduction ratio TAU A′ is less than the second thickness reduction ratio TAU A″.

The thickness reduction ratio TAU A is defined as the difference between the thickness H of the cast product P exiting from the crystallizer 15 a and the slab thickness SB2 exiting from the casting machine 11, correlated to the thickness H of the cast product P exiting from the crystallizer 15 a.

According to a possible solution of the present invention, the Applicant has determined that the casting machine 11 is selectively set to exert an action of reducing the thickness of the cast product P, defined by the formula:

TAUA = K × A × H¹ × e^(a × SN)

In which:

-   K: is a variable parameter between 0.8 and 1.1; -   A: is a coefficient equal to about 4689; -   H: is the thickness of the cast product P exiting from the     crystallizer 15 a, that is the thickness of the crystallizer 15 a; -   a: is a coefficient equal to -0.37; -   SN: is the thickness of the strip N to be obtained at the end of     rolling.

The TAU A which derives from the formula is a percentage value and, by way of example only, can be a value normally comprised between 2% and 75%.

FIG. 10 shows some examples of curves, determined in relation to the thickness H of the cast product P, which show the development of the TAU A as a function of the strip thickness SN.

This formula allows to optimize the setting of the casting machine 11, as a function of the thickness of the crystallizer 15 a and of the strip thickness SN to be obtained for an efficient functioning of the entire production plant, as defined above.

According to a possible solution of the invention, the casting machine 11 is configured to exert:

-   i) an action of reducing the thickness of the cast product P by     means of liquid core pre-rolling with the roller unit 16, referred     to as thickness reduction ratio in the roller unit TAU1; -   ii) an action of reducing the thickness of the cast product P by     means of the action of the drawing unit 18, referred to as thickness     reduction ratio in the drawing unit TAU2.

The thickness reduction by means of the drawing unit 18 can be optional.

The thickness reduction ratio TAU A is therefore given by the combination of the reductions TAU 1 and TAU 2.

The thickness reduction ratio in the roller unit TAU 1 is defined by the difference between the thickness H of the cast product P exiting from the crystallizer 15 a and the slab thickness SB1 exiting from the roller unit 16, correlated to the thickness H of the cast product P exiting from the crystallizer 15 a.

The thickness reduction ratio in the drawing unit TAU2 is defined as the difference between the slab thickness SB1 exiting from the roller unit 16 and the slab thickness SB2 exiting from the drawing unit 18, correlated to the slab thickness SB1 exiting from roller unit 16.

According to a possible solution of the present invention, the Applicant has determined that the drawing unit 18 is selectively set to exert an action of reducing the thickness SB1 of the solid core cast product P, defined by the formula:

TAU2 = Q × (B × H^(b) × ln SN + C × H^(c))

In which:

-   Q: is a variable parameter between 0.8 and 1.1; -   B: is a first coefficient equal to 10928; -   H: is the thickness of the cast product P exiting from the     crystallizer 15 a as defined above; -   b: is a second coefficient equal to -1.659; -   SN: is the thickness of the strip; -   C: is a third coefficient equal to 10648; -   c: is a fourth coefficient equal to -1.596.

In accordance with a possible solution, the solid core reduction of the thickness of the cast product P is applied for strip thicknesses SN comprised between 0.6 mm and 3.5 mm. In fact, for these sizes of strip thickness SN, the action of liquid core reduction is relatively high in order to reduce, right from the start, the starting slab to the maximum, and therefore it is advantageous to also couple, with the action of liquid core reduction, a further action of solid core reduction of the thickness exerted with the drawing unit 18.

Furthermore, the action of solid core rolling exerted with the drawing unit 18 allows to increase the production of the strip, and this is particularly advantageous for thin strip thicknesses.

FIG. 11 shows some examples of curves, determined in relation to the thickness H of the cast product P exiting from the crystallizer, which show the development of the TAU2 as a function of the strip thickness SN.

The reduction ratio in the roughing unit 12 is called TAU3 and is defined as the difference between the thickness SB2 of the slab B upstream of the roughing unit 12 and the thickness of the slab B downstream of the roughing unit 12, correlated to the thickness SB2 of the slab B upstream of the roughing unit 12.

The reduction ratio in the finishing unit 21 is called TAU4 and is defined as the difference between the thickness of the slab B upstream of the finishing unit 21 and the strip thickness SN obtained, correlated to the thickness of the slab B upstream of the finishing unit 21.

The overall reduction ratio of the rolling station 19 is defined as TAUB and is defined as the difference between the slab thickness SB2 exiting from the casting machine 11 and the thickness of the strip N obtained, correlated to the slab thickness SB2 supplied by the casting machine 11. The overall reduction ratio TAUB of the rolling station 19 can also be defined as the combination of the reduction ratios of the roughing unit 12 and of the finishing unit 13, TAU 3 and TAU 4.

In accordance with a further aspect of the present invention, the method to produce metal strip comprises the supply, by a client, of design data comprising at least:

-   a productivity PR, for example annual, of the production plant 10, -   a range of thicknesses RS and an average width LN of strip that the     production plant 10 will have to produce; -   the respective distribution of the productivities as a function of     the thicknesses of strip N produced; -   the types of products that can be cast with the casting machine,     that is, the steel grades as above; -   the respective distribution of the productivities as a function of     the types of castable products.

The set of parameters above defines the so-called “product mix” of the plant, that is, the variety of products that the production plant 10 will have to produce and according to which it is sized in order to achieve the purposes of the present invention.

Supplying the range of thicknesses RS provides to determine a minimum thickness SMIN and a maximum thickness SMAX of strip obtainable.

In some embodiments of the present invention, the rolling method provides to determine the optimal type of rolling mode of the slab B in relation to the product mix requested by the client.

The rolling mode of the slab B is chosen from endless, semi-endless and coil to coil rolling modes.

FIG. 12 shows a graph showing the criteria for choosing one of the rolling modes as above, at least in relation to the strip thicknesses SN and to castable steel grades.

In general, to make a production plant 10 that allows to work with several operative modes, the rolling mode to be implemented on each occasion will be defined by optimizing the operating conditions of the plant, on the basis of economic assessments (energy consumption and yield of the process) and of the quality required for the final product (size tolerances and mechanical characteristics of the strip).

By way of example only, it can be provided that if in the product mix there is a prevalence of thin thicknesses without special types of steels, for example peritectic steels which are difficult to cast and require low casting speeds, an endless type of rolling mode is chosen, and therefore one of the plant layouts shown in FIGS. 7, 8 or 9 can be chosen.

If in the product mix there is a prevalence of thin thicknesses but there is also a need to cast special types of steels, a coil to coil and semi-endless rolling mode is generally chosen, and therefore one of the plant layouts shown in FIGS. 6 and 9 can be chosen. Both the coil to coil and semi-endless modes allow to cast special steels at low speed, since the casting process is not constrained by the rolling process. The semi-endless mode is preferred for thin and ultra-thin strip N thicknesses, for example comprised between 0.8 mm and 1.4 mm. The coil to coil mode is preferred instead for strip thicknesses greater than 1.4 mm.

FIG. 13 graphically shows a further criterion for choosing the rolling modes in relation to the strip thickness and the capacity of the plant layout chosen to adopt one and/or the other of the rolling modes as above.

By way of example only, with reference to FIG. 13 , if the constructive layouts of FIGS. 2-5 are chosen, it is provided to make the plants work in coil to coil mode for strip thicknesses comprised between about 1.2 mm and about 12 mm, while if the constructive layout of FIG. 9 is chosen, it is provided to make the plant work in endless mode for strip thicknesses comprised between about 0.6 mm and about 1 mm, in semi-endless mode for strip thicknesses comprised between about 1 mm and about 2 mm and in coil-to-mode for strip thicknesses comprised between about 2 mm and about 12 mm.

According to a further solution, the production method provides to set a casting speed VC which is selected from a value comprised between 4.5 m/min and 6 m/min.

In particular, for types of steel that are difficult to cast, such as API, peritectic and Corten steels, a lower casting speed is advantageously set, for example comprised between 4.5 and 5 m/min. For steels that are easier to cast, such as Low Carbon, Medium Carbon, HSLA, DP, CP, HC, MnB steels, a higher casting speed is set, for example comprised between 5 m/min and 6 m/min.

In the same way, the choice of casting speed VC can also be defined in relation to the rolling modes selected, that is, relatively low casting speeds for coil to coil and semi-endless modes, and higher casting speeds for endless modes.

The production method then provides to determine a nominal slab thickness SBN value which, in relation to the casting speed VC and to the average strip width LN, allows to reach the productivity PR.

The nominal slab thickness SBN can also be interpreted as the constant thickness of an equivalent slab which, for the same overall production for that given product mix, corresponds to the average of the (variable) slab thicknesses after the liquid core reduction weighted (the average) on the respective hourly productions.

In accordance with a possible solution, the nominal slab thickness SBN is determined by the formula: SBN=PR/OPERATING HOURS/(VC*LN*PS)

By operating hours, here and in the following description and claims, we mean the functioning hours, that is, when the plant is operating, in the calendar year, net of production stoppages due to maintenance, accidents or suchlike.

Wherein PS is the specific weight of steel which usually can be around 7.8 kg/dm³.

With reference to FIGS. 14 and 15 , graphs are shown that correlate the nominal slab thickness SBN, to the annual productivity PR of the plant and to the casting speed VC.

In particular, FIG. 14 refers to a width of the slab of about 1300 mm, while FIG. 15 refers to a width of the slab of about 1400 mm.

According to a possible implementation of the present invention, the method comprises determining the thickness H of the cast product P exiting from the crystallizer 15 a, that is, determining the distance between the plates of the crystallizer 15 a in correspondence with the exit section of the latter. This thickness H of cast product P is the one adopted for the entire product mix defined by the client.

In a possible implementation of the invention, the thickness H of the cast product P exiting from the crystallizer 15 a is a number comprised between 10 mm and 15 mm more than the nominal slab thickness SBN.

The method then provides to determine a ratio of thicknesses RSP applied by the rolling station 19 and which is calculated as the ratio between the value of the slab thickness SB2 entering the rolling station 19, when the strip N of minimum thickness SMIN is processed, and the minimum thickness SMIN of the strip N obtainable.

The value of the slab thickness SB2 entering the rolling station 19 when the strip N of minimum thickness SMIN is processed is calculated as the thickness H of the cast product P exiting from the crystallizer 15 a minus the maximum thickness reduction imparted by the casting machine. According to a possible solution, this maximum thickness reduction induced by the casting machine 11 is at least equal to 31 mm, or more.

Subsequently, the method provides to determine the number of rolling stands of the rolling station 19, in relation to the ratio of thicknesses.

For a ratio of thicknesses comprised between 4 and 12, four rolling stands are provided.

For a ratio of thicknesses comprised between 12 and 21, five rolling stands are provided.

For a ratio of thicknesses comprised between 21 and 52, six rolling stands are provided.

For a ratio of thicknesses comprised between 52 and 110, seven rolling stands are provided.

This correlation between the number of rolling stands and the ratio of thicknesses is shown in the graph of FIG. 16 .

The number of rolling stands of the rolling station 19 is intended as the sum of the number of finishing stands 21 and the number of roughing stands 20.

Subsequently, the method provides to set a mode to distribute the liquid core reduction and the solid core reduction in the casting machine 11, performed with the roller unit 16 and with the drawing unit 18, on the slab B exiting from the crystallizer 15 a, in relation to the final thickness of the strip N.

This mode to distribute the reduction in the thickness of the cast product P can be determined by means of the TAU A and TAU 2 formulae defined above.

In accordance with a further possible solution, the casting machine 11 can be set to perform a reduction in the thickness of the cast product P equal to 5 mm for a strip thickness equal to the maximum thickness SMAX, and a thickness reduction comprised between 25 mm and 31 mm for a strip thickness equal to the minimum thickness SMIN.

According to a possible solution, the roller unit 16 can perform a maximum liquid core reduction RCLMAX of about 25 mm. According to a possible solution, the roller unit 16 can perform a minimum liquid core reduction RCLMIN of about 5 mm. This reduction allows to confer greater quality to the cast metal product P.

According to one solution of the invention, the drawing unit 18 can perform a maximum solid core reduction of at least 7 mm, while the minimum reduction is zero.

In particular, each individual pair of rollers of the drawing unit 18 can achieve, when necessary, a compression comprised between 0.5 mm and 1.5 mm.

In accordance with possible implementations of the method, if a coil to coil type production mode is set, and the use of a winding/unwinding device 29 is provided, the number of the roughing stands 20 is determined so as to supply at entry to the winding/unwinding device 29 a bar with a thickness comprised between 8 mm and 25 mm. Bar thicknesses of less than 8 mm would entail problems with transport and introducing the bar into the winding/unwinding device, while bar thicknesses greater than 25 mm would lead to operating and/or sizing difficulties with possible generation of surface defects on the bar itself.

In accordance with further possible implementations of the method, if a coil to coil type production mode is set, and the use of a heating unit 33 is provided, the number of the roughing stands 20 is determined so as to supply at entry to the heating unit a bar with a thickness greater than or equal to 30 mm. Sizes smaller than these thickness values would require the use of an extremely long heating unit, difficult to manage and uneconomical.

According to a possible solution, it is provided that the number of finishing stands 21 is at least two.

EXAMPLES

With reference to FIGS. 17-22 , two examples of implementation of the teachings of the present invention are now described.

Specifically, with reference to FIGS. 17-19 , a first case is shown in which a production PR of approximately 1.1 Mton/year, a range of thicknesses RS comprised between 1.2 mm and 8 mm and an average width LN of the strip of about 1300 mm are required.

With reference to FIGS. 20-22 , on the other hand, a second case is shown in which a production PR of approximately 1.4 Mton/year, a range of thicknesses RS comprised between 0.8 mm and 3 mm and an average width LN of the strip of about 1400 mm are required.

With reference to the tables shown in FIGS. 17 and 20 it is possible at the very least to observe in the last four columns on the right a comparison of the distribution of productivity in relation to the product mix in the case of a conventional casting and in the case of a casting with thickness reduction according to the teachings of the present invention.

The product mix and annual productivity values (see the eleventh and twelfth column of the tables) are parameters requested by the client and set according to how the client needs to use the plant.

Also with reference to the tables shown in FIGS. 17 and 20 , it is possible to observe how in the sixth column the slab thickness SB2 exiting from the casting machine 11 is not a constant value as the thickness of the strip varies, but it is a value that decreases as the thickness in the strip decreases. In the conventional or known technique, on the other hand, the slab thickness SBN exiting from the casting machine is always the same as the thickness of the strip varies, as shown in the second column from the left. This consideration is also shown graphically in FIGS. 18 and 21 .

This variation in the thickness of the slab in relation to the thickness of the final strip is obtained by the fact that the casting machine 11 imparts on the cast product a liquid core reduction RCL of the thickness which is shown in column three of the tables of FIGS. 17 and 20 .

The tables also show the thickness reductions performed with the drawing unit 18 (fifth column), with the roughing unit 12 (seventh column), and with the finishing unit 13 (ninth column), and how these actions are distributed according to the final strip thicknesses to be obtained.

FIGS. 19 and 22 , on the other hand, graphically show the distribution of the hourly productivities of the plant as a function of the strip thickness to be obtained, in the case of a known conventional casting, and of a casting with variable thickness reduction in accordance with the present invention.

In particular, in both FIGS. 19 and 22 it can be noted that, in the case of a known conventional casting, the hourly productivity is kept unchanged for any strip thickness to be obtained, while with the casting in accordance with the present invention the hourly productivity is varied in relation to the strip thicknesses to be obtained.

From an analysis of FIGS. 19 and 22 it can be observed that for low strip thickness values the present invention allows a lower productivity compared to conventional solutions, while for high strip thickness values the present invention provides a higher productivity than conventional solutions. Overall, however, the annual productivities of the plant in the case of conventional casting and casting in accordance with the present invention are equal to each other, and in the solution according to the present invention it is possible to obtain the advantages above compared with the known solution.

With reference to the first case shown in FIGS. 17-19 , we will now describe a mode for choosing the type of plant and the number of rolling stands of the rolling station 19.

In particular, on the basis of the range of variable strip thicknesses between 1.2 mm and 8 mm, and in relation to the graph in FIG. 12 , it is advantageous to adopt a plant layout able to implement a coil to coil type rolling mode.

It is then provided to set a casting speed of about 5.5 m/min, to which, for an average strip width LN of 1300 mm and a productivity of 1.1Mton/year, corresponds a nominal slab thickness SBN of about 45 mm, as can also be derived graphically from FIG. 14 .

On the basis of this SBN value it is possible to determine the thickness H of the cast product P which is between 10 mm and 15 mm greater than the nominal slab SBN, value in this case equal to 55 mm, as shown in the table in FIG. 17 .

It is then provided to determine the thickness reduction to be imparted by the rolling station 19.

The slab thickness SB2 entering the rolling station 19 when the minimum thickness is processed is 24 mm.

The ratio of thicknesses RSP of the rolling station 19 is therefore 24/1.2=20 overall.

With reference to FIG. 16 it is possible to observe that a number of stands equal to 5 corresponds to this ratio of thicknesses.

With reference to conventional casting, on the other hand, the conventional ratio of thicknesses is given by the ratio between the nominal slab thickness SBN and the minimum thickness SMIN of the strip, in this case 45/1.2=37.5. With reference to FIG. 16 , a number of stands equal to 6 corresponds to this ratio of thicknesses. From this example it is possible to observe how, with the same annual productivity of the plant, it is possible, with respect to the conventional production mode, to reduce the number of rolling stands by one unit.

With reference to the second case shown in FIGS. 20-22 , we will now describe another mode for choosing the type of plant and the number of rolling stands of the rolling station 19.

In particular, on the basis of the range of variable strip thicknesses between 0.8 mm and 3.0 mm and in relation to the graph of FIG. 12 , it is advantageous to adopt a plant layout able to implement an endless rolling mode.

It is then provided to set a casting speed of about 6.0 m/min to which, for an average width LN of 1400 mm and a productivity of 1.4Mton/year, corresponds a nominal slab thickness SBN of about 50 mm, as can also be derived graphically from FIG. 15 .

On the basis of this SBN value it is possible to determine the thickness H of the cast product P which is between 10 mm and 15 mm greater than the nominal slab SBN value, in this case equal to 65 mm, as shown in the table in FIG. 20 .

It is then provided to determine the ratio of thicknesses to be imparted by the rolling station 19.

The slab thickness SB2 entering the rolling station 19 when the minimum thickness is processed is 34 mm.

The ratio of thicknesses RSP of the rolling station 19 is therefore 34/0.8=42.5 overall.

With reference to FIG. 16 it is possible to observe that a number of stands equal to 6 corresponds to this ratio of thicknesses.

With reference to conventional casting, on the other hand, the conventional ratio of thicknesses is given by the ratio between the nominal slab thickness SBN and the minimum thickness SMIN of the strip, in this case 50/0.8=62.5. With reference to FIG. 16 , a number of stands equal to 7 corresponds to this ratio of thicknesses. From this example it is possible to observe how, with the same annual productivity of the plant, it is possible, compared with the conventional production mode, to reduce the number of rolling stands by one.

It is clear that modifications and/or additions of parts may be made to the method to produce a metal strip, and to the production plant that implements this method as described heretofore, without departing from the field and scope of the present invention.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of method to produce a metal strip, and of production plant that implements this method as described heretofore, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

In the following claims, the sole purpose of the references in brackets is to facilitate reading: they must not be considered as restrictive factors with regard to the field of protection claimed in the specific claims. 

1. A method to produce metal strips, in particular steel, in a production plant configured to work in coil to coil and/or semi-endless rolling mode and/or in endless rolling mode, in which through a casting machine provided with a crystallizer is cast a slab, with a defined casting speed Vc, and said slab is hot rolled in a rolling station, comprising a number of rolling stands, to obtain metal strip having different strip thicknesses, wherein said casting machine, during casting, exerts a first action of liquid core reduction of the thickness of the slab exiting from the crystallizer, through a pre-rolling device with rollers, and a second action of solid core reduction of the thickness of the slab, through a drawing unit located downstream of said pre-rolling device, wherein said casting machine is selectively set, as said strip thicknesses vary, in order to exert a different action of reducing the thickness of the slab, in which said first action of liquid core reduction of the thickness is comprised between 5 and 25 mm, said second action of solid core reduction of the thickness is maximum 7 mm and is applied for strip thicknesses comprised between 0.6 mm and 3.5 mm, with an overall reduction thickness of the slab exiting from the casting machine comprised between 2% and 75%, and wherein the rolling mode of the slab is chosen in relation to said strip thicknesses and to the various types of produced steel, in order to optimize the productivity of the strip production plant with the lowest number of stands possible, while maintaining the maximum casting speed Vc in relation to the various types of produced steel.
 2. The method as in claim 1, wherein said casting machine comprises a pre-rolling device with rollers which exerts an action of reducing the thickness of the cast product exiting from the crystallizer by means of pre-rolling with liquid core.
 3. The method as in claim 1, wherein said pre-rolling device with rollers is provided with a plurality of opposite rollers between which said cast product passes and said rollers are selectively moved toward/away from each other to exert a selective action of liquid core reduction of the thickness of said cast product.
 4. The method as in claim 2, wherein said pre-rolling device with rollers is provided with a plurality of opposite rollers between which said cast product passes and said rollers are selectively moved toward/away from each other to exert a selective action of liquid core reduction of the thickness of said cast product.
 5. The method as in claim 1, wherein said casting machine comprises a drawing unit which exerts an action of reducing the thickness of the cast product exiting from said pre-rolling device with rollers, by means of solid core rolling of the cast product.
 6. The method as in claim 2, wherein said casting machine comprises a drawing unit which exerts an action of reducing the thickness of the cast product exiting from said pre-rolling device with rollers, by means of solid core rolling of the cast product.
 7. The method as in claim 3, wherein said casting machine comprises a drawing unit which exerts an action of reducing the thickness of the cast product exiting from said pre-rolling device with rollers, by means of solid core rolling of the cast product.
 8. The method as in claim 4, wherein said casting machine comprises a drawing unit which exerts an action of reducing the thickness of the cast product exiting from said pre-rolling device with rollers, by means of solid core rolling of the cast product.
 9. The method as in claim 1, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 10. The method as in claim 2, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 11. The method as in claim 3, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 12. The method as in claim 4, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 13. The method as in claim 5, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 14. The method as in claim 6, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 15. The method as in claim 7, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 16. The method as in claim 8, further comprising determining the thickness of said cast product exiting from the crystallizer which is a number comprised between 10 mm and 15 mm more than a nominal slab thickness (SBN) defined by the formula SBN = PR/OPERATING HOURS/(VC*LN*PS), where PR is the productivity of the plant, VC is a casting speed chosen between 4.5 m/min and 6 m/min, LN is the average width of the strip, and PS is the specific weight of the steel.
 17. The method as in claim 1, further comprising determining a thickness ratio (RSP) applied by the rolling station calculated as the ratio between the value of the slab thickness (SB2) entering said rolling station, when the strip of minimum thickness (SMIN) is processed, and the value of said minimum thickness (SMIN) of the strip.
 18. The method as in claim 2, further comprising determining a thickness ratio (RSP) applied by the rolling station calculated as the ratio between the value of the slab thickness (SB2) entering said rolling station, when the strip of minimum thickness (SMIN) is processed, and the value of said minimum thickness (SMIN) of the strip.
 19. The method as in claim 17, further comprising determining the number of rolling stands of the rolling station, where: for a ratio of thicknesses comprised between 4 and 12, four rolling stands are provided; for a ratio of thicknesses comprised between 12 and 21, five rolling stands are provided; for a ratio of thicknesses comprised between 21 and 52, six rolling stands are provided; for a ratio of thicknesses comprised between 52 and 110, seven rolling stands are provided.
 20. A plant for producing metal strip, configured to work in coil to coil and/or semi-endless rolling mode and/or in endless rolling mode, comprising a casting machine provided with a crystallizer for casting a slab, with a defined casting speed Vc, and a rolling station for hot rolling of said slab, the rolling station comprising a number of rolling stands, to obtain metal strip having different strip thicknesses, wherein said casting machine, during casting, exerts a first action of liquid core reduction of the thickness of the slab exiting from the crystallizer, through a pre-rolling device with rollers, and a second action of solid core reduction of the thickness of the slab, through a drawing unit located downstream of said pre-rolling device, wherein said casting machine can be selectively configured, as said strip thicknesses vary, in order to exert a different action of reducing the thickness of the slab, in which said first action of liquid core reduction of the thickness is comprised between 5 and 25 mm, said second action of solid core reduction of the thickness is maximum 7 mm and is applied for strip thicknesses comprised between 0.6 mm and 3.5 mm, with an overall reduction thickness of the slab exiting from the casting machine comprised between 2% and 75%, and wherein the rolling mode of the slab is chosen in relation to said strip thicknesses and to the various types of produced steel, in order to optimize the productivity of the strip production plant with the lowest number of stands possible, while maintaining the maximum casting speed Vc in relation to the various types of produced steel. 